The present invention is directed to biaxially oriented polymeric films. Specifically, the present invention is directed to biaxially oriented polyethylene films having unidirectional tear characteristics in machine direction and methods thereof.
Polymeric films are widely used in many industrial applications. One particularly important application is the food packaging industry. Films employed in the food packaging industry are chosen to provide characteristics necessary for proper food containment. Polymeric films used for food packaging, however, usually do not have tear propagation in the film plane in one well-defined direction. For example, non-oriented polymeric films such as polypropylene (PP) or polyethylene (PE) films are difficult to tear in any direction. If a tear is made on a package of non-oriented polymeric film, the tear propagates in an uncontrolled way across the package, and destroys the entire package.
Bioriented high density polyethylene (HDPE) or polypropylene films have easy tear propagation in both machine and transverse directions. Packages produced with these bioriented polyolefins are easy to open if a tear is initiated by a notch in the pack (typically in the seal area). However, these films are not without disadvantages. Tear propagation of these films is uncontrolled. That is, if a tear is made on a package of the polymeric films, the tear propagates in an uncontrolled way across the package and the entire package is destroyed.
MD oriented polymeric films have also been developed to give controlled tearing properties in one direction. For example, Monax films manufactured by Tredegar provide controlled tear in machine direction. However, the production of MD oriented film is limited in efficiency, as only relative narrow web width can be produced and thickness uniformity is generally poor.
In addition, oriented films that give controlled tear in transverse direction are disclosed in U.S. Pat. No. 4,870,122, issued Sep. 26, 1989 to Lu; U.S. Pat. No. 4,916,025, issued Apr. 10, 1990 to Lu; and U.S. Pat. No. 5,223,346, issued Jun. 29, 1993 to Lu. These films have controlled opening in transverse direction, and thus have, unfortunately, inherent sensitivities in web resistance against tearing when the film is produced, converted, or used in packaging applications.
As a result, there exists a continuing need for bioriented films having controlled tear properties, while maintaining uniform optical properties, good mechanical properties in both machine and transverse directions, high production output with a large web to assure good material efficiencies, and uniform thickness.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits excellent unidirectional tear properties in the machine direction.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits uniform optical properties.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits good mechanical properties in both machine and transverse directions and uniform film thickness.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits high production output with a large web to assure good material efficiencies.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description and its scope will be pointed out in the appended claims.